This invention relates generally to surface acoustic wave (SAW) devices and, more particularly, to SAW devices used in the analysis of linear-frequency-modulated (LFM) or chirp signals.
SAW devices employ substrates of a piezoelectric material, across which elastic surface waves are propagated between sets of electro-acoustic transducers disposed on the substrate surface. The surface waves, called Rayleigh waves, have an amplitude of displacement that is largest right at the substrate surface. In a piezoelectric material, deformations induced by the waves induce local electric fields, which are propagated with the acoustic waves and extend into space above the surface of the material. These electric fields will interact with electrodes disposed on the surface of the material, to serve as electrical input and output transducers for the surface acoustic wave device.
There is a need in some applications to detect signals that have been subject to unknown linear frequency modulation (LFM), sometimes referred to as chirp signals. It is important in these applications to be able to determine the chirp rate, that is the time rate of frequency change, df/dt, usually measured in megahertz per microsecond. Prior approaches to this problem include the use of SAW reflective array compressors (RACs). The detection of each particular chirp rate requires the use of a separate RAC. Since most applications involve the detection of multiple chirp rates, multiple RACs are needed, and this arrangement is relatively bulky and expensive.
Accordingly, there is a need for a simpler form of SAW device to perform the same function. As will become apparent from the following summary, the present invention satisfies this need.